The present invention relates to position encodement and detection for servo systems used in disk files employing continuous servo information for track-seeking and track-following operations to enable high track densities. The invention is described herein in connection with a magnetic disk file, but it is usable in other environments as well. Early magnetic recording disk files employed open loop positioning systems for controlling the movement of one or more magnetic heads to different concentric recording tracks on a magnetic disk or disks. With the demand for increased track density, closed loop servo systems were employed utilizing a separate disk surface dedicated to track position information, as first commercially exemplified by the IBM 3330 disk file.
In some disk files, particularly in lower cost files, sector servo systems have been utilized, in which sectors of track position information are interspersed around the disk surface between adjacent data areas. While such sector servo systems have the advantage that the servo information resides on the same disk surface as the data tracks on which track-following is to take place, they have the disadvantage that they inherently reduce the area available for recording data.
It has long been recognized that a servo system in which the servo information and data coexist in a disk would be desirable. Even though such techniques have been known to be desirable, they have never been exploited, apparently because of the mutual interference between signals from the so-called buried servo and signals from data tracks, or the expense of providing a multiple layer magnetic coating, one for the position-indicating servo signals and another for the data signals.
An early teaching of superposed data and servo signals is found in U.S. Pat. No. 3,404,392. This patent teaches using a dual layer magnetic coating on a disk substrate, a lower layer having a higher retentivity and coercivity than the upper layer. A servo signal is first recorded in the lower layer for indicating track positions, and following such recording, data signals are recorded. During the operation, both servo and data signals are sensed.
A later development is shown in the IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 1974, beginning at page 536. This shows a single magnetic coating on a substrate, and teaches that the ratio of the frequency of the data signals to the servo signal should be 10:1, and that the servo signal can be successfully recorded with a head having sufficient ampere-turns capability.
U.S. Pat. No. 3,765,005 teaches using a clock-bias signal at the upper end of a data signal frequency spectrum. Also, U.S. Pat. No. 3,821,798 shows a low frequency control signal recorded at a lower end portion of the data signal pass band.
A later reference showing buried servo is U.S. Pat. No. 3,956,769, which teaches the use of two separate gaps in a transducer; one for servo signals and one for data signals.